1. Field of the Invention
The present invention relates generally to rotary bits for drilling subterranean formations. More specifically, the invention relates to fixed cutter or so-called xe2x80x9cdragxe2x80x9d bits which are fabricated from steel, known as steel body bits, employing superabrasive cutters and tailored structural elements substantially fabricated from hardfacing materials.
2. State of the Art
Hardfacing has been used in the downhole tool art for some time as a way to increase the erosion and abrasion resistance of certain areas of roller cone bits and steel body bits. Relatively thin layers of hardfacing have been applied to relatively large areas where erosion and abrasion from cuttings, high-velocity fluid and contact with the formation causes undesirable wear on the bit. Steel bits, such as roller cone bits, exhibit much more erosive and abrasive wear than so-called matrix bits which are manufactured by infiltration of molten metal into a matrix material comprising tungsten carbide or other powder. Many fixed cutter drill bits are manufactured from tungsten carbide matrix, as well as from steel. Steel body bits tend to exhibit superior toughness but limited erosion and abrasion resistance, whereas matrix bits tend to exhibit reduced toughness but exemplary erosion and abrasion resistance.
Hardfacing is generally composed of some form of hard particles delivered to a surface via a welding delivery system. Hardfacing refers to the deposited material rather than the constituent materials which make up the hardfacing. Constituent materials of hardfacing are referred to as a hardfacing composition. Hard particles may come from the following group of cast or sintered carbides consisting of chromium, molybdenum, niobium, tantalum, titanium, tungsten, and vanadium and alloys and mixtures thereof, as disclosed by U.S. Pat. No. 5,663,512 to Schader et al., assigned to the assignee of the present invention and incorporated by reference herein. Commonly, a mixture of sintered, macrocrystalline, or cast tungsten carbides is captured within a mild steel tube. The steel tube containing the carbide mixture is then used as a welding rod to deposit hardfacing onto the desired surface, usually with a deoxidizer, or flux.
The shape, size, and relative percentage of different hard particles will affect the wear and toughness properties of the deposited hardfacing, as described by Schader et al. U.S. Pat. No. 5,492,186 to Overstreet, assigned to the assignee of the present invention and incorporated by reference herein, describes a hardfacing configuration for heel row teeth on a roller cone drill bit. The coating comprises two hardfacing compositions tailored for different properties. A first hardfacing composition may be characterized by good sliding wear resistance and/or abrasion resistance with a lower level of toughness. The second hardfacing composition contains carbide particles of spherical sintered, crushed sintered and cast tungsten carbide. A substantial portion of the particles in the second composition are characterized by a higher level of fracture resistance, or toughness, and a lower level of abrasion resistance.
Hardfacing compositions have been also used for coating the gage surfaces of roller cone teeth, as disclosed in U.S. Pat. No. 3,800,891 to White et al. White also discloses, with respect to the hardfacing of teeth on a milled steel tooth rolling cone-type bit, circumferential grooves and a transverse slot on each roller cone tooth for the deposition of hardfacing.
Hardfacing has been utilized with steel body bits in certain circumstances. For example, U.S. Pat. No. 4,499,958 to Radtke et al. discloses hardfacing on the blades and other portions of the bit subject to abrasive wear. However, use of hardfacing material as taught by Radtke et al. does not address issue of material toughness as may be required for various portions of the bit while also exploiting the advantages of an abrasion-resistant material.
So-called matrix bits, aforementioned for their superior abrasion and erosion resistance, have also been contemplated as benefitting from hardfacing as well. U.S. Pat. No. 4,884,477 to Smith et al., assigned to the assignee of the present invention, discloses a metal matrix bit body composed of a filler material of higher toughness than tungsten carbide with substantially all of the internal and external surfaces of the bit body coated with an erosion- and abrasion-resistant hardfacing comprised of tungsten carbide or silicon carbide. However, Smith et al. does not address strategic localization of a material according to its characteristics of either abrasion resistance or material toughness. Smith et al. fails to particularly address such issues with regard to a steel body bit.
Additionally, while many efforts have been directed at utilizing and improving hardfacing and its application to drill bits, multiple hardfacing compositions have not been used to enhance or form structural elements on steel body drill bits. For example, structural elements of a steel body drill bit which substantially protrude from the surface of the drill bit, such as wear knots or chip breakers, have not previously benefitted from the use of hardfacing materials.
Wear knots may serve to limit the depth of cut of cutting structure on a drill bit during operation and thereby protect the cutting structure from damage. Wear knots for steel body drill bits may be conventionally formed by press fitting a sintered tungsten carbide stud into a hole milled into the bit body. Alternatively, a wear knot may be machined into the bit body, although this requires a predetermination of the placement of the wear knot and may limit the design topography of the drill bit.
Chip breakers serve to influence the formation of chips which are initiated at the leading edges of cutters and are pushed along the surface of a blade of the bit carrying the cutters such that they are weakened and subsequently broken into smaller elements during the drilling process. Such a chip breaker is described in greater detail in U.S. Pat. No. 5,582,258 to Tibbitts et al., assigned to the assignee of the present invention and incorporated by reference herein. Chip breakers form a xe2x80x9cbumpxe2x80x9d in the surface of the blade and in the direct path of the formation of the chip which causes the chip to break before becoming overly elongated. This breakage prevents chips from building up along the surface of the bit and possibly balling the bit with an agglomeration of chips, as is known in the art. Chip breakers in steel body bits may be machined into the surface of the bit; however, this too may place limits on the bit design.
Gage elements for steel body bits are typically formed by drilling holes into the gage surface and pressing sintered tungsten carbide cylinders into the holes. As an additional measure, a layer of hardfacing may be applied around the sintered carbide cylinders, on the body of the bit, but the cylinders function as the main elements to prevent abrasion and wear on the gage, and are designed and configured to maximize the exposed area of the sintered cylinders to the borehole sidewall. Although sintered carbide cylinders function adequately as a drill bit gage, the necessity of milling precise holes for press fitting is cumbersome and limits the configuration of the gage. In addition, sintered carbide gage cylinders often exhibit cracking after use, referred to as crazing, perhaps attributable to the extreme heating and cooling cycles present during drilling conditions.
In view of the shortcomings in the art, it would be advantageous to provide a steel body drag-type bit employing structurally protruding elements formed of hardfacing materials. It would further be advantageous to provide hardfacing in a drill bit wherein such hardfacing was localized according to the material properties of the hardfacing material. Such localization could be employed to include hardfacing of multiple material compositions exploiting advantageous material properties of each individual composition.
It would also be advantageous to provide a method of modifying existing bits to employ structurally protruding elements formed of a hardfacing material. Such a method would allow for the simpler and more cost-efficient manufacture of such bits while still allowing for application-specific customization of such bits.
It would also be advantageous to provide a bit, as well as a method of manufacturing such a bit, exhibiting a tailored surface with respect to the manner in which hardfacing is applied such that a desirable stress state is imparted to the resultant hardfacing structure. It would be advantageous to employ hardfacing having such a resultant stress state designed according to the expected loading or stress imparted to the bit while in operation.
The inventors herein have recognized that structural elements of a steel body drill bit may be formed by application of hardfacing. Modifying surface geometry of the surface receiving the hardfacing and modifying hardfacing compositions are techniques of tailoring the structural elements according to the present invention.
Specifically, according to one aspect of the invention, a gage is formed by applying one composition of hardfacing to rotationally leading and trailing edges of the gage pad and filling in between these edges on the radially outer surface of the gage pad with a second different hardfacing composition. This allows for tailoring of the hardfacing properties for each respective area. By way of example, if the edges are expected to experience an increased amount of chipping, the hardfacing composition in that area may be tailored with respect to toughness. In the area between the edges, where cracking may be less of a concern, the hardfacing composition may be tailored with respect to wear characteristics.
Another aspect of using multiple hardfacing compositions in different places along the bit applies to the use of hardfacing as a protective coating. As such, multiple materials may be used to coat the outer surfaces of the drill bit to hinder erosion and abrasion. For example, where more erosion-resistant materials are needed, a hardfacing with a relatively large amount of macrocrystalline tungsten carbide may be used. Similarly, for example, where hardfacing with increased toughness is desired, spherical sintered and cast tungsten carbide may be used. In the degenerate case, the entire surface of applied hardfacing on the steel body drill bit would be tailored, area by area, with desired characteristics. More practically, selected areas would be tailored for desired hardfacing characteristics as needed.
In accordance with yet another aspect of the invention, a gage is defined by forming grooves in a gage pad of a steel bit body and subsequently filling the grooves with a hardfacing composition. The grooves are believed to reduce chipping of the hardfacing during drilling of a subterranean formation. Also, the grooves provide an increased amount of surface area for attaching the hardfacing to the bit body as well as an increased volume of hardfacing. Hardfacing compositions may be varied as well, as described in the first embodiment, where a first hardfacing is used on rotationally leading and trailing edges and a second hardfacing is used in between the two rotational edges on the radial outer surface of the gage pad. In a further combination, grooves may be located in various regions along the surface of the gage.
Carried further, the grooves may be oriented and tailored for loading and residual stress considerations. Orienting the grooves generally along the longitudinal axis of the blade is one configuration; however, it may be beneficial to orient the grooves with respect to loading characteristics of the blade. In addition, it is contemplated that a beneficial stress-relieved state in the hardfacing may be achieved by modifying the surface of the gage to which hardfacing is applied via at least one groove. This stress state will manifest as a result of thermal expansion differences between the bit body material and the hardfacing upon affixing the hardfacing to the bit at a high temperature. Compressive stress states are generally preferable for brittle materials; however, tensile stress states may be advantageous as well. Overlapping grooves, grooves with different depths, concentric grooves, V-shaped grooves, U-shaped grooves, or otherwise configured or combined groove geometries may be used to achieve a desired result.
The present invention also contemplates forming wear knots or chip breakers on a steel body bit. Several advantages are apparent from this method. For example, a bit may be manufactured without wear knots or chip breakers initially, and then, if wear knots or chip breakers are desired, the bit may be subsequently configured with wear knots or chip breakers fabricated from a hardfacing material. This expands the suitability of one bit for multiple applications. Also, in the case of a worn bit, modifications and repairs to the wear knots or chip breakers are easily made when provided from hardfacing materials, as opposed to conventional techniques of creating these structures.
Stated another way, the present invention encompasses and includes the overall concept of providing protruding hardfacing structures on steel body bits such as wear knots and chip breakers, as well as gage pads and protective coatings formed from at least two different hardfacing compositions. Additionally, the invention encompasses and includes steel body drill bit surfaces comprising at least one groove for accepting hardfacing.